1. Field of the Invention
The present invention relates to a resonator, filter, duplexer, and high-frequency circuit apparatus used in the microwave or millimeter wave band for use in radio communication or in electromagnetic-wave transmission/reception.
2. Description of the Related Art
In the related art, U.S. Pat. No. 6,148,221 (the '221 patent) discloses a resonator incorporating a multilayer thin-film electrode.
The multilayer thin-film electrode disclosed in the '221 patent is formed by alternately layering conductive thin films and dielectric thin films, and serves as an electrode which provides low loss in a high-frequency region. In a design method disclosed in the publication, the optimum thicknesses of the conductive thin films and the dielectric thin films depend upon the conductivity and the dielectric constant, respectively. Optimizing the thicknesses of the conductive thin films and the dielectric thin films allows the current density to be uniformly distributed over the layered conductive thin films, thereby mitigating the skin effect. The multilayer electrode can therefore be operated with lower loss than a single-layer electrode.
In the resonator disclosed in the '221 patent which incorporates a multilayer thin-film electrode, the dielectric constant and thickness of the dielectric thin films are adapted to control a displacement current between the conductive thin films in order to distribute a current substantially uniformly over the conductive thin films of the multilayer thin-film electrode. Thus, the following two requirements are essential for low-loss operation of the multilayer thin-film electrode:    (1) that the multilayer thin-film electrode be orthogonal to the orientation of electric field vector; and    (2) that the dielectric constant and thickness of the dielectric thin films be designed to be optimum or close to optimum.
In the resonator disclosed in the '221 patent, therefore, a single-layer electrode is used for an electrode tangential to the orientation of electric field vector, and ends of each of the thin conductive layers of the multilayer thin-film electrode formed on the surface orthogonal to the orientation of the electric field vector are short-circuited by the single-layer electrode. Otherwise, the surface tangential to the orientation of the electric field vector is open, and no electrode is formed on that surface.
FIGS. 16A and 16B are a top plan view and a front view, respectively, of an open-circuited circular TM010-mode resonator in the related art. FIG. 16C is a cross-sectional view showing an enlarged part of the resonator shown in FIG. 16B. In FIGS. 16A to 16C, a multilayer thin-film electrode 10 having a two-layer construction in which a dielectric thin film 3 is sandwiched between conductive thin films 2a and 2b is formed on each of two parallel surfaces of a cylindrical dielectric member 1.
FIGS. 17A and 17B are a top plan view and a front view, respectively, of a short-circuited circular TM010-mode resonator. FIG. 17C is a cross-sectional view showing an enlarged part of the resonator shown in FIG. 17B. In FIGS. 17A to 17C, the peripheries of conductive thin films 2a and 2b are connected to a single-layer conductive film 4 so that the peripheries of the conductive thin films 2a and 2b may be short-circuited.